In wireless communication systems, the use of antenna arrays at the base station has been shown to increase both range, through increased gain, and capacity, through interference suppression. With adaptive antenna arrays, the signals received by multiple antenna elements are weighted and combined to improve system performance, e.g., by maximizing the desired receive signal power and/or suppressing interference. The performance of an adaptive antenna array increases dramatically with the number of antennas. Referring to an article entitled, "The Impact of Antenna Diversity on the Capacity of Wireless Communication Systems," by J. H. Winters, R. D. Gitlin and J. Salz, in IEEE Trans. on Communications, April 1994, it is shown that using an M element antenna array with optimum combining of the received signals can eliminate N .ltoreq.M-1 interferers and achieve an M-N fold diversity gain against multipath fading, resulting in increased range.
Most base stations today, however, utilize only two receive antennas with suboptimum processing, e.g., selection diversity where the antenna having the larger signal power is selected for reception and processing. It is desirable to be able to modify existing base stations to accommodate larger arrays of antennas and/or improved received signal combining techniques. However, modifying existing equipment is difficult, time consuming, and costly, in particular since equipment currently in the field is from a variety of vendors.
One alternative is to utilize an applique, which is an outboard signal processing box, interposed between the current base antennas and the input to the base station, which adaptively weights and combines the received signals fed to the base station, optionally utilizing additional antennas. FIG. 1 shows a base station utilizing an applique. A key to the viability of utilizing the applique approach is that it should require little, if any, modification of the base station equipment. This constraint implies that the processing performed by the applique must be transparent to the existing equipment. Ideally, the signal emerging from the applique should appear to the existing base station as a high-quality received signal from a single antenna.
A difficulty in obtaining transparency to the existing equipment is in the weight calculation. With the correct symbol timing and carrier frequency, the weights can be generated to combine the signals received from multiple antennas to increase gain and suppress interference, permitting operation even with noise and/or interference power that is greater than the signal power. However, before the adaptive array combining, the desired signal may be masked by noise and interference. When the desired signal is so masked, traditional symbol timing and carrier frequency recovery methods will not work.
A previously proposed solution is scanning a narrow beam antenna coupled with utilizing traditional symbol timing and carrier frequency recovery methods as the beam is scanned. However, this approach will not work when the interference is in the same narrow beam as the desired signal and in severe multipath environments.
Therefore, there is a need to calculate the weights for combining the received signals which will function in severe multipath and interference environments.